Phys AS unit 2 section 7.3 - moments

physics AS unit 2 section 7.3 moments
In this section we seek to answer the questions:

• why do we call turning forces 'moments'?
• how do you calculate the turning force around a pivot?
• why do we need both the strange words 'pivot' and 'fulcrum' to describe the same thing?
• what is a centre of mass?
• how do you 'pop a wheelie' in a '73 Camero?

A Fast and Furious introduction to moments:






Where is the physics/mathematics in this clip? What questions are you asking yourself?

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You probably remember moments from your earlier studies of science.
You should know that the turning moment of a force F applied distance d from a pivot (fulcrum) is given by F×d.

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Ok let's see a little more...








What data do you have so far?
Do you need to see that again?
What else do you need to know?
Why?

So here's some more information...





How did Matt calculate the clockwise moment about the rear axle?
What did he assume? Is that reasonable?


Let's check your understanding.

• Work out the anticlockwise (counter-clockwise) moment.
• Here's the data you need:

Now check your answer:

• Were you right?
• Reflect on the values for the turning moments you've found so far.
• The car is at rest, so it's not going anywhere. In particular it's not rotating.
• What can you say about the moments when an object is in equilibrium (and hence at rest)?
• Where is the missing moment that keeps the car from rotating coming from?
• Can you estimate the force there?
• As the car accelerates, the engine applies torque to the rear axle. This is another turning moment which reduces the force you've just described.
• So when does the car wheelie?

We're ready to set the problem:

• how do they do that?

There at least four possible ways to solve the problem facing the stunt designer.

• What are they?
• Which one is most feasible/desirable from the point of the movie maker?
• Exactly how did they achieve the stunt?
• Give values and show your assumptions and calculations.

Watch the last clip to see if your answer matches Matt Parker's:



still don't believe it can be done? check this out:


You may wish to refine Matt's model using this data from a real '73 Camaro:


Now for some more practice: